feat: add sentinel plate and plate list enhancements

Always keep a trailing empty plate so users can immediately place parts
without manually adding a plate. Auto-appends a new sentinel when parts
land on the last plate; trims excess trailing empties on removal.

Plate list now shows Parts count and Utilization % columns. Empty plates
are filtered from save and export. Sentinel updates are deferred via
BeginInvoke to avoid collection-modified exceptions and debounced to
prevent per-part overhead on bulk operations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/Api/NestResponsePersistenceTests.cs

@@ -13,6 +13,9 @@ public class NestResponsePersistenceTests
     {
         var nest = new Nest("test-nest");
         var plate = new Plate(new Size(60, 120));
+        var drawing = new Drawing("test-part");
+        nest.Drawings.Add(drawing);
+        plate.Parts.Add(new Part(drawing));
         nest.Plates.Add(plate);
 
         var request = new NestRequest